Multimetal corrosion-resistant diffusion coatings

ABSTRACT

Methods and compositions are provided herewith for the production of multimetal diffusion coatings on metal articles providing prolonged protection against chemical or galvanic corrosion of the surface of the coated article during prolonged exposure to corrosive conditions, and particularly high-saline content marine atmospheres, especially where the protective coating is also subjected to mechanically erosive and abrasive environments, with the multiplicity of coating metals being selected so that the combination thereof provides a coating varying through the thickness thereof from outer surface toward the interface of coating layer and coated article so that the mechanical resistance to chemical corrosion is greatest at the outer surface but decreases as the coating thickness is eroded away, while the components of the coating offering sacrificial or cathodic protection are more concentrated adjacent the coatingarticle interface so that cathodic protection of the coated articles increases as the coating is removed by abrasion or erosion. As illustrative, the outer coating surface includes a high concentration of metallic components inherently resistant to saline corrosion and/or abrasion, although offering less cathodic protection for the coated article; while inner layers of the coating are rich in metallic components offering high sacrificial or cathodic protection, although less erosion or saline corrosion resistance.

United States Patent Brill-Edwards 1 Feb. 15,1972

[54] MULTIMETAL CORROSION- Chromalloy American Corporation, West Nyack,N.Y.

[22} Filed: May3l, 1968 [21] Appl.No.: 733,303

[73] Assignee:

[52] U.S. Cl ..29/l96.5, 29/197 [51] Int. Cl ..B32b 15/00 [58] Field ofSearch ..29/196.5, 197

[56] References Cited UNITED STATES PATENTS 2,023,364 12/1935 Crapo..29/l96.5 2,296,838 9/1942 Domm ..29/ 196.5 3,064,337 11/1962 Hammond..29/196.5 3,323,881 l/l967 Nelson ..29/196.5 3,438,754 4/1969Shepard... ..29/196.5 3,079,276 2/1963 Puyear ..29/197 PrimaryExaminer-Hyland Bizot Att0mey-Hopgood & Calimafde [57] ABSTRACT Methodsand compositions are provided herewith for the production of multimetaldiffusion coatings on metal articles providing prolonged protectionagainst chemical or galvanic corrosion of the surface of the coatedarticle during prolonged exposure to corrosive conditions, andparticularly high-saline content marine atmospheres, especially wherethe protective coating is also subjected to mechanically erosive andabrasive environments, with the multiplicity of coating metals beingselected so that the combination thereof provides a coating varyingthrough the thickness thereof from outer surface toward the interface ofcoating layer and coated article so that the mechanical resistance tochemical corrosion is greatest at the outer surface but decreases as thecoating thickness is eroded away, while the components of the coatingoffering sacrificial or cathodic protection are more concentratedadjacent the coating-article interface so that cathodic protection ofthe coated articles increases as the coating is removed by abrasion orerosion. As illustrative, the outer coating surface includes a highconcentration of metallic components inherently resistant to salinecorrosion and/or abrasion, although offering less cathodic protectionfor the coated article; while inner layers of the coating are rich inmetallic components offering high sacrificial or cathodic protection,although less erosion or saline corrosion resistance.

3 Claims, No Drawings M ULTIMETAL CORROSION-RESISTANT DIFFUSION COATINGSThis invention relates to the protection of metal articles fromcorrosion in highly saline and/or marine atmospheres despite the factthat such articles are also to be used under circumstances where severemechanical erosion or abrasion of the protective coating is likely to beexperienced and, more particularly, to multicomponent diffusion coatingsin which the extent of sacrificial protection of the coated article fromcomponents in the coating increases as the thickness of the protectivecoating is eroded away by mechanical abrasion by utilizing for the outersurface of the coating components which have marked erosion andcorrosion resistance, although little potential for cathodic protection,while utilizing in inner layers of the coating adjacent the coatedarticle components with a high potential for cathodic protectionalthough being of reduced erosion or corrosion resistance.

As purely illustrative of certain applications to which this inventionis particularly related, and the problems incident thereto, one may notethe circumstances and environments incident to the operation of certaincomponents of jet aircraft engines in highly saline atmospheres (such aslow flying aircraft, particularly helicopters, adjacent seaports andseacoasts), particularly where the highly saline marine atmosphere alsoincludes substantial amounts of erosive particles of sand, coral dust,etc. Whereas the anticorrosion problems of turbine components (ascompared to compressor components) of such jet aircraft enginessubjected primarily to only the impingement of extremelyhigh-temperature combustion gases may relate so much to oxidationresistance at such high temperatures that other possible sources ofcorrosion become insignificant, the compressor components of such jetengines experience quite different problems. For example, the compressorcomponents, while rarely subjected to operating temperatures above 900F., are subjected to direct impingement of highly saline atmospheres atthe air intake, which may also include substantial amounts of abrasiveparticulate material, in addition to being subjected to tremendousmechanical stresses from centrifugal forces, thermal shock, vibration,etc., and under circumstances (particularly with single-engine aircraft)where premature failure of the compressor parts for whatever reason maybe catastrophic, regardless of the fact that the greater problem ofextremely high-temperature oxidation resistance of hot turbinecomponents may have otherwise been solved.

Thus, if the particular materials (such as high-strength ferrous alloyslike those designated in the Aerospace Material Specifications of theSociety of Automotive Engineers as AMS 5508, AMS 5615, AMS 6304, etc.)are utilized for their mechanical strength in the compressor, they mayhave inadequate saline resistance or erosion resistance, without someprotective surface treatment, and to an extent which cannot assure auseful and failure-free life comparable to that of the high-temperaturerefractory metals or superalloys utilized in the hot turbine componentsof the same jet engines.

But jet engine compressor components must function and maintain theirfunctioning within very low tolerances and clearances which do not admitof a substantial buildup of corrosion products (rust, pulverous oxides,etc.), and even miniscule pitting from saline corrosion (characteristicof virtually all stainless steel parts subjected to saline atmospheres)may reduce the mechanical strength factor many fold and to an extentwhich is inimical with adequate performance in such highly stressedparts as compressor blades, compressor rotors and, particularly, thejunctures therebetween subjected to great centrifugal and othervibrational and mechanical forces in a rotating manner and at speedswhich cannot tolerate substantial radial imbalances.

If it is attempted to protect against the saline corrosion to beexpected in such applications in marine atmospheres by, for example, adiffused nickel-cadmium electroplated protective coating (as set forthin AMS procedure 2416 D) or by the diffusion coating of metallic zincinto the surface of the ferrous structures to be protected, less thanoptimum results may be encountered. For example, considering anickel-cadmium coating in which the nickel is adjacent the ferroussubstrate (as is almost inevitable because cadmium is virtuallyinsoluble in iron and, thus, is difficult satisfactorily to diffuse intoa ferrous surface), neither component is sufficiently anodic relative tothe substrate to assure absolute sacrificial protection in local areaswhere the electroplated nickel has not adhered to the ferrous substrate;and, furthermore, when the surface layer of cadmium is removed orpenetrated by the inevitable erosion or abrasion encountered in suchspecific applications as noted above, the exposed sublayer of nickel iselectrochemically noble relative to the ferrous substrate so that salinegalvanic corrosion is actually intensified to an extent even greaterthan that which would occur in the same atmosphere with uncoated ferroussubstrates.

Similarly, relying on a protective diffusion coating of zinc mayeliminate or curtail the exaggeration of electrochemical salinecorrosion of the ferrous substrate, but zinc is itself highly corrosivein saline atmospheres producing voluminous quantities of white rustdeposit (such as complexes of zinc hydroxide and zinc chloride) whichbuild up on the surface of the parts and diminish the mechanicaltolerances and aerodynamic effectiveness of the surfaces thereof.

As will be understood, the foregoing difficulties are not necessarilydisastrous in various types of hardware, even for the aforementionedapplications. For example, the blades and shroud vanes of a jet enginecompressor may be considered much more susceptible to mechanical erosionor abrasion by airborne sand or coral dust particles than is thecompressor rotor disc around the periphery of which the blades arefastened. Yet all are subjected to whatever electrochemical corrosion isinherent in such saline environments, and it is believed unrealistic toattempt to design a chemically or electrochemically or cathodicallyprotective coating without recognizing that it may be subjected tomechanical erosion or abrasion onslaught at a rate which far exceeds theexpected life of the coating even if it be considered as whollyelectrochemically sacrificial. By the same token, if the protectivecoating be subjected both to mechanical erosion and sacrificial cathodicprotection, particularly under circumstances where the rate of erosionexceeds that of cathodic sacrifice, then the need for cathodicprotection increases the more the thickness of the coating diminishes byerosion-at least to the extent that the less the coating mechanicallyprotects against erosion and corrosion (i.e., the thinner the coatingbecomes), the greater is the need for cathodic protection of thesubstrate.

In accordance with this invention, by contrast to previous suggestedsolutions of such problems, there is provided a multicomponent diffusioncoating for ferrous metal articles or substrates having a compositiongradient through the thickness of the coating from outer surface tocoating-substrate interface varying in chemical and mechanicalproperties whereby, regardless of the resistance of the coating layersto mechanical erosion or abrasive wear, the outer surface hasconcentrated therein components particularly resistant to salinecorrosion (although of less potential for cathodic protection of thesubstrate), while inner portions of the coating adjacent the substrateto be protected have concentrated therein components of increasedpotential for cathodic or sacrificial protection of the substrate(although perhaps less than maximumly resistant to either mechanicalerosion or direct chemical corrosion by the saline atmosphere). Yet allsuch components are applied in accordance therewith, and integrated withthe metal article substrate to be protected by diffusion techniquesproducing Substantially complete integration of the various coatingcomponents with the substrate article to be protected in a manner whichsubstantially eliminates mechanical cracking or separation of thecoating relative to the substrate despite intense mechanical or thermalshock stresses which may be encountered in use. As a further feature ofthis invention, there are provided a plurality of techniques forapplying such a multicomponent protective coating, either in one or morestages of application, to provide segregated but metallurgically unifiedareas through the coating of varying compositions and specificallylocated with regard to the nobility, sacrificial potentials, cathodicprotection characteristics, and corrosion and erosion resistances toachieve optimal mechanical and electrochemical or cathodic protection ofthe metal article being coated.

With the foregoing and additional objects in view, this invention willnow be described in more detail, and other objects and advantages willbe apparent from the following description and the appended claims.

In attempting to select and devise the appropriate combinations ofmultimetallic systems for achieving adequate or optimum protectivecoatings in accordance herewith, a number of independently variable, andoften inconsistent, criteria need be kept in mind. For example, in orderto avoid mechanical failures or separation under thermal shock orphysical cracking of the coating materials, it is preferred that anactual diffusion of the coating material into the substrate be obtained(as compared, for example, to merely an electrodeposition or plating,although that may be an appropriate means for preliminary deposition ofthe coating metal provided it can be subsequently diffused into thesubstrate). Yet, there are some metals which might provide desiredelectrochemical protection but which are not susceptible to actualdiffusion into a ferrous substrate (cadmium, for example, is virtuallyinsoluble in iron and, thus, not susceptible for solid state diffusionthereinto).

Similarly. some metals are available for good initial corrosionprotection or resistance to corrosion in saline atmospheres andadequately diffusable into iron, but which do not render the desiredcathodic or sacrificial protection of iron in a saline atmosphere(nickel, for example, which, being more noble than iron, actuallyincreases the iron corrosion in any area where the nickel protectivecoating may have been worn away sufficiently to expose the ironsubstrate surface). Other metals which may be readily diffusable intoiron and electrochemically appropriate for sacrificial protectionthereof may not be sufficiently initially resistant to saline corrosionor mechanical erosion to last as a protective coating long enough fortheir sacrificial protection to be realized (zinc, for example, isreadily diffused into ferrous substrates and sacrificially protectivethereof, but so susceptible to initial saline corrosion as to producesuch large initial quantities of white rust" as frequently to outweighthe ultimate or subsequent protection which might be afforded).

Nevertheless, considering the foregoing criteria, and others as will benoted hereinafter, multicomponent protective coatings have been devisedin accordance herewith to give satisfactory or enhanced protection underthe saline-corrosive and mechanically erosive environments noted byutilizing a combination of various metals to form a multilayer coatingwhere all components are diffused or interdiffused with a ferroussubstrate and under circumstances where the composition gradient throughthe coating is such as to maximize initial saline corrosion andmechanical erosion when the coated article is put into use, while latermaximizing cathodic sacrificial protection as outer layers of thecoating are eroded or corroded away.

Merely as illustrative of the teachings hereof, one such protectivecoating system may be noted as a combination of cadmium and zincsupplied as a diffusion coating over the surface of a ferrous substrateunder such circumstances that approximately the outer one-third of thecoating thickness is rich in cadmium, while the inner two-thirds is richin zinc. In this manner, initial saline corrosion of zinc is minimizedby having the cadmium outer layer, while the cadmium-rich outer layerproduces far less saline corrosion products than does zinc. Althoughcadmium is virtually insoluble in iron (thus minimizing its utility fordirect diffusion coating into an iron substrate), it is readily solublein zinc, and zinc is readily soluble in iron, so that the combinationprovides for an integrated diffusion coating of zinc into the ironsubstrate and the cadmium into the zinc coating.

It is not, however, necessary that the process be carried out in theabove two-stage fashion. For example, satisfactory results were obtainedin accordance herewith in the protective coating of steel articlesfabricated from a ferrous material such as AMS 5616 (characterized inthe Aerospace Material Specifications of the Society of AutomotiveEngineers as steel including about 13 percent chromium, 2 percentnickel, and 3 percent tungsten) by heating the articles to be coated atabout 730 F. for about 12 hours embedded in a powder pack including, byweight, about 10 percent metallic zinc powder (-325 mesh), about 10percent cadmium powder (325 mesh) with a balance of about percenttabular alumina as an inert filler. After such treatment, the articles,still embedded in the pack, were additionally heated to a temperature ofabout 830 F. for an additional half hour, and then air cooled. Becauseof the ready solubility of zinc into iron and the lack of solubility ofcadmium, the foregoing process results in a protective coating with theouter surface primarily cadmium and the inner portion adjacent thecoating-substrate interface rich in zinc in a manner which provides fora cadmium-zinc protective outer surface and a large reservoir of zincadjacent the substrate interface for cathodic sacrificial protection ofthe ferrous article when ultimate erosion of the outer surface of thecoating occurs.

Somewhat similar results were obtained in accordance herewith in atwo-stage pack impregnation process by first producing a preliminarycoating of zinc (about 0.8 mil thick and comprising approximately 13percent iron and 87 percent zinc) on the ferrous article to be coated byembedding it in a powdered pack including about 10 percent zinc, avaporizable halogen or halide energizer (0.25 percent ammonium iodideand 0.125 percent iodine), and alumina as an inert filler, and heatingat about 830 F. for 12 hours in a closed retort (all in known andwell-understood manner). Thereafter the thus coated article was embeddedin a powdered pack comprising about 3.4 percent zinc powder (325 mesh),16.6 percent cadmium powder (325 mesh) and the balance tabular aluminafor an additional coating cycle at about 730 F. for l2 hours to providean outer cadmium coating layer integrally and diffusion bonded with theferrous article and the preliminary zinc coating.

Although, as noted above, electroplating of the protective coating maynot give optimum results because of a lack of integrity which maydevelop between the electroplated layer and the substrate, suchtechniques are appropriate for the preliminary deposition of variouscomponents of the protective coating in accordance herewith providedthey can be subsequently integrated by diffusion or interdiffusion ofthe various components. For example, satisfactory results have beenachieved in accordance herewith by diffusion coating a zinc layer on aferrous substrate (such as AMS 5616) by the technique noted above(embedding in a pack comprising alumina, 10 percent zinc powder, 025percent ammonium iodide and 0.125 percent iodine for a heating cycle of12 hours at 830 F.) and thereafter electroplating cadmium over thethuscoated ferrous article from a conventional cyanide electroplatingbath. Thereafter, the plated article was heated in an inert powderedpack at about 650 F. for 4 hours to promote diffusion of the platedcadmium layer into the zinc-coated surface of the ferrous article forultimate diffusion integration of the several components of theprotective coating and to provide a cadmium-rich outer surface underwhich was a zincrich sacrificially protective layer bonded to theferrous substrate.

As will be understood from the disclosure thereof, cadmium-zinccombinations are particularly attractive for multicomponent protectivecoatings in accordance herewith because both such metals have high vaporpressures at relatively low temperatures and thus satisfy one of therequirements for direct vapor deposition in a powder pack cementationprocess. Furthermore, the vapor pressures of these two metals areadequately high for satisfactory deposition at treating temperatureswhich do not adversely affect the mechanical properties of the hardenedand tempered steels of the types referred to earlier from which theferrous metal articles are likely to be fabricated for applications towhich this invention is particularly related (e.g., coating temperaturesless than 900 F.).

Furthermore, the cadmium-zinc combination is particularly interesting inthis connection because the physical vapor phase deposition of a metal(as opposed to a volatile compound of that metal) in a thermallyhomogenous powdered cementation pack is appropriate if the metaldissolves in the substrate to form an alloy therein the vapor pressureof which at coating temperature is lower than that of the metal beingdeposited. Although cadmium does not bear this relationship with iron,zinc does behave in this manner with respect to the ferrous substrate,and cadmium is soluble enough in zinc to be satisfactorily depositedsimultaneously with zinc into a ferrous substrate and/or into a ferroussubstrate previously coated with zinc.

Additionally, as noted above, these two metals are particularlyattractive regarding the erosion and sacrificial protection of ferrousarticles because zinc is highly cathodically protective of ferroussubstrates (although not itself sufficiently resistant to direct salinecorrosion) and cadmium is highly resistant to saline corrosion (althoughnot itself sufficiently sacrificial to protect fully a ferrous substrateas with zinc, and the cadmium-zinc combination is more resistant tomechanical abrasion than either component alone. Similarly, the relativesolubilities of cadmium and zinc in either zinc or iron automaticallycontrol the concentration gradient through the coating so as toconcentrate cadmium on the outer surface while the inner layer of thecoating adjacent the substrate interface is primarily rich insacrificially protective zinc.

As will be apparent from the foregoing, a diffusion coating having acadmium-rich surface and a zinc-rich undercoat is capable of affordingadequate cathodic protection of the ferrous substrate while, at the sametime, avoiding severe surface corrosion or white rust formation in ahighly saline atmosphere. As the other (cadmium) surface becomespartially or wholly removed by erosion or abrasion so as to reduce thecoating thickness and even locally expose the substrate, then thezinc-rich undercoat thus exposed will afford increased cathodicsacrificial protection in the weakened areas. Even under suchconditions, white rust deposits from the zinc form only in those areaswhere the zinc undercoat is exposed by erosion, not over the entirecoating surface, and additional protection against saline corrosion andpitting of the ferrous substrate is extended by the cathodic orsacrificial protection of the zinc increasingly at the end of theexpected service life of the coating where such protection is mostcrucial as the thickness of the coating becomes more and more eroded tothe point of ultimate necessary replacement, repair, or recoating of theferrous article.

Cadmium-zinc coatings of the type described above have been tested in asalt spray cabinet in accordance with Federal Test Method No. 81 1.15 lato assess the protectability thereof, and in all such official testing,as well as other experience evaluations, the protectiveness of suchcoatings and the prolonged useful life thereof has far exceeded resultswith other previously acceptable zinc or cadmium or nickel coatingsutilized for similar applications as discussed above.

Considering another multicomponent coating system with whichsatisfactory results are achieved in accordance herewith, one may note atechnique involving first coating the ferrous substrate with a diffusioncoating of zinc, thereafter electroplating a thin layer of nickel ontothe zinc-coated surface, and thereafter additionally applying adiffusion coating of zinc with interdiffusion of all three layers fordiffusion bonding to the ferrous substrate and for driving the outerzinc layer into and through the nickel layer so as to provide an outercoated surface rich in nickel (for initial corrosion and abrasionresistance) and an inner layer rich in zinc for sacrificial protectionupon eroding of the outer layer. In accordance with this embodiment, theferrous article is first coated by impregnation in a zinc-containingcementation pack, as noted above, for 12 hours at 830 F after which athin layer of nickel is electroplated onto the thus-coated surface byany of known means such as utilizing a conventional nickel sulfamatebath employing a current density of about 0.4 amps per square inch and aplating time of about 4 minutes. Thereafter the plated article is againembedded in the zinc cementation powdered pack for an additional 2 hoursheating at about 830 F.

The above-noted treatment cycles are particularly designed to facilitateinterdiffusion and bonding between all layers of the coating and betweenthe zinc undercoat and the substrate.

' In this way, an efiicient metallurgical bond is assured at allcompositional interfaces, thereby reducing the possibility of layerspalling in use. The coating produced in accordance with this embodimenthas a composition profile or a concentration gradient through thecoating such that the zinc content of the coating drops from about 60percent at a point approximately one-third of the way into the coating,and then rises to a constant level of about 88 percent over the innerone-third of the coating. The nickel content, on the other hand, risessteadily from the surface to reach a maximum (corresponding to the zincminimum) at about one-third of the coating, and thereafter decreasestoward the substrate-coating interface at which point it approximateswhatever is the original nickel content (if any) of the base metal beingcoated.

Although a substantially pure zinc diffusion coating was applied as thelast step to the outer surface of the above example, the diffusionconditions of the application of the zinc layer are such as to cause thezinc to diffuse into and through the nickel layer, thus uniting theelectroplated nickel and decreasing the surface content of zinc. Forthis reason, the third step involves a difi'usion coating of zinc for nomore than about 2 hours, because a heavier application of zinc wouldresult in too high zinc concentration on the outer surface and, thus,have little or no advantage over merely a zinc coating over the entirearticle. Indeed, for many applications, the first zinc coating step maybe eliminated, with satisfactory results in accordance herewith havingbeen achieved by directly electroplating nickel on the ferrous article,and then applying an outer zinc diffusion coating from a cementationpack with diffusion of the zinc into and through the electroplated layerto bond it to the surface of the ferrous article but still maintainingthe concentration gradient of high nickel content at the surface andhigh zinc content for sacrificial protection adjacent thecoating-substrate interface. Satisfactory results have been achieved byapplying the zinc diffusion coating over the electroplated nickel from apack such as noted above at a temperature of 750850 F. for about 12hours. The presence of highly sacrificial zinc between the outer nickellayer and the ferrous substrate eliminates the above-noted difficultieswith cadmium-nickel systems whereby erosion of the coating and exposureof adjacent areas of nickel and ferrous substrate actually increaseselectrochemical corrosion because of the fact that nickel is noble withrespect to ferrous metals in saline atmospheres and, consequently, notcathodically sacrificially protective thereof.

As further illustrative of multimetal coating systems embodying and forpracticing this invention may be noted the diffusion coating obtainedwith satisfactory results on AMS 6304 steel utilizing a combination ofzinc and lead. Such coatings were produced from a single-stagecementation process using a powdered pack including, by weight, 5percent lead, 5 percent zinc, 90 percent tabular alumina, and 0.5percent ammonium iodide, in which pack the AMS 6304 articles were heatedat about 900 F. for 20-60 hours. The resulting coating provided a highconcentration of lead at the outer surface, with a preponderance of zincand very little lead in the undercoating adjacent the substrate surface.Here again, the surface concentration of lead decreases the immediatesaline corrosion of zinc (with the undesirable white rust formationcharacteristic of such corrosion), while the internal concentration ofzinc affords enhanced sacrificial protection when the outer surface ofthe coating begins to show the effect of mechanical erosion or abrasion.

In saline atmospheres, lead is considerably less anodic than zinc, asdesired in accordance herewith, and relatively insoluble in the iron ofthe ferrous substrate, but soluble in the alloy produced between theferrous substrate and zinc.

As will be apparent from the foregoing examples, satisfactory resultshave been achieved with coatings on steel materials and using thezinc-cadmium, zinc-nickel, or zinc-lead systems as disclosed, byproducing the coatings at treating temperatures no higher than about 900F. or less. With the types of steel-base articles noted above, as iswell understood, a significant change or deterioration in metallurgicaland mechanical properties (particularly fatigue life, tensile strength,and thermal shock resistance) may occur with metal articles fabricatedfrom such materials if the articles are subjected to a postfabricationtemperature treatment above 900-l,000 F. When the continued maintenanceof high mechanical properties is an important consideration in thearticle being coated (as with the jet aircraft engine compressorcomponents referred to above), it is of some significance to be able toprovide whatever postfabrication surface coating may be desired forcorrosion protection at coating or treating temperatures below those atwhich the base metal article itself undergoes undesired crystallographicor metallurgical transformations, and it is in this area, among others,where the corrosion-resistant coatings according to this invention areparticularly advantageous, in addition to the above-noted quality ofproviding preliminary corrosion and erosion protection at the surfacewith increased cathodic sacrificial protection as the coating isinevitably worn away.

It has been possible to devise diffusion coating systems andaccelerators therefor which permit the diffusion coating of metal suchas aluminum at coating temperatures far below those conventionallyrequired for aluminum-base diffusion coatings (as disclosed in copendingapplication Ser. No. 733,287, now U.S. Pat. No. 3,589,935 filed of evendate herewith) to avoid the above-noted metallurgical difficulty, andsome of such systems (notably those combining aluminum and zinc) may beconsidered as having an electrochemical potential in saline atmosphereswhich would be cathodically sacrificially protective of ferroussubstrates. Similarly, as disclosed in copending publication Ser. No.733,286, filed of even date herewith, other aluminum-base diffusioncoatings are provided to provide cathodic protection of ferroussubstrates in saline atmospheres and at relatively low coatingtemperatures. In some of such other types of coatings, moreover, thereis also to be found the additional feature here whereby the outersurface of the coating is rich in metal preliminarily resistant tosaline corrosion and mechanical erosion, while inner layers of thecoating are rich in a metal providing cathodic protection of thesubstrate although, perhaps, itself deficient in direct or initialcorrosion resistance in the saline atmosphere against which protectionis desired.

For example, utilizing a pack comprising percent aluminum, 1 percenteach cadmium and zinc, 0.5 percent ammonium iodide, and 0.25 percenturea, with the balance being tabular alumina, AMS 6304 articles werecoated at 900 F. during a -hour heating cycle to produce coatings ofabout 1 mil in thickness and having the composition of about 1-10percent zinc and 60-65 percent aluminum at the substratecoatinginterface, the balance being iron. The zinc content decreased to zeroaway from the substrate-coating interface, while the aluminum contentincreased toward the surface, thus producing the desired concentrationgradient in accordance herewith whereby initial protection of the outersurface from both corrosion and erosion is attributable to the aluminum,while a zinc concentration near the substrate produces a potential forcathodic protection as the thickness of the coating is eroded orcorroded away in use. Nevertheless, as noted below, the total potentialdifference between such coatings and substrate may not be as great aswith the zinc-cadmium coatings in accordance herewith, but, with certainsubstrate materials, the aluminum-zinc coatings are sufficiently anodicto give the desired cathodic protection as the coating is worn thinnerin use.

Also, as will be apparent from the foregoing, the coating systemsembodying and for practicing this invention utilize the combination of ahighly sacrificial metal of greater solubility in the substrate and aless sacrificial metal of less solubility in the substrate, and,preferably, also greater resistance to preliminary corrosion anderosion, with, of course, the further admonition that the two metalsmust be compatible with each other in the coating environment andsusceptible of being diffusion coated as desired. It may also be notedthat the more sacrificial ofthe two elements should possess anelectrochemical potential with respect to the substrate sufficientlyhigh to provide the desired cathodic protection thereof in salineatmospheres, but, preferably, no higher than is practically necessary todo so.

That is, if the more sacrificial coating metal is unnecessarily highlyanodic with respect to the substrate, the principal result will be thatit will be sacrificed, when electrochemical corrosion commences, at anunnecessarily high rate (however much it may protect the substrate),thus ultimately decreasing the total service life of the wholeprotective coating. As illustrative of the foregoing, one may note somecomparative electrode potential data regarding a number of the foregoingillustrative examples.

The electrode potentials of a number of samples were measured in a 3percent NaCl solution at 22 C. with reference to a standard saturatedcalomel electrode (E=0.242 volts on the hydrogen scale) after 5 minutesof immersion in the salt solution, with the specimens being cylinders 1inches long and 7/16 inches in diameter. As will be understood, the moreanodic surface was indicated by a higher negative electrode potential,which indicates a greater tendency for the surface metal to dissolve inor be corroded by the saline environment or atmosphere. Merely asillustrative of data obtained in such determinations, it may be notedthat uncoated specimens of AMS 5616 and 6304 showed potentials of,respectively, 0.29 v. and -O.57 v., with such substantial variationsapparently being attributable to the fact that the higher proportion ofchromium in AMS 5616 substantially lowers the negative potential.

An AMS S616 specimen with a diffusion coating of zinc alone showed apotential of O.92 v., thus confirming the tremendous propensity for thezinc to combine with the saline atmosphere, however cathodicallyprotective the zinc might be for the ferrous substrate. It was foundthat such high negative potential of the zinc coating could besubstantially reduced (e.g., down to O.83 v.) by chemical conversion ofthe diffused coating by immersion of the coated specimens in a bath ofzinc phosphate in known manner, but the results of such conversion,particularly in atmospheres producing excessive erosion or mechanicalabrasion, may be short lived. A specimen coated with a zinc-cadmiumcoating in accordance herewith (produced by the single-stage packcementation process noted above) gave a potential of0.8l v.

Similarly, regarding AMS 6304 specimens, a diffusion coating of zincalone had a potential of 0.9l v., which was reduced to O.82 v. bychemical conversion in a zinc phosphate bath. A zinc-cadmium diffusioncoating in accordance herewith and produced by a single-stage packcementation process had potential of 0.81 v. Additional specimens of AMS5616 and 6304 coated with the aluminumzinc diffusion coating noted abovehad potentials of, respectively, 0.78 v. and -O.77 v., which werefurther somewhat reduced by chemical conversion of the coated surface inaluminum chromate solutions (such as Alodine) to give, respectively,O.74 v. and O.72 v.

As will be apparent from the foregoing data, all the various coatingsindicated are considerably more anodic than the ferrous substrate, thusoffering sacrificial protection. Nevertheless, the pure zinc coatingsexhibit such high negative potential that corrosion thereof in salineatmospheres would be excessively rapid, even if there were not theadditional disadvantage of the excessive formation of white rust"deposits on the article. Even reducing this negative potential byadditional chemical treatment of the coated article produces noparticular advantage over the zinc-cadmium coatings in accordanceherewith, lacks the permanence thereof due to the interdiffusion of bothmetals, and requires an additional treatment step, while lacking theconcentration gradient through the coating which provides for erosionand corrosion resistance at the coated surface and increased sacrificialprotection nearer the substrate as the coating thickness becomes erodedaway.

It has been developed in accordance herewith that adequate sacrificialprotection of the substrate is obtained if the negative potential of theprotective coating is at least 0.2-0.3 v. higher than that of thesubstrate. Such a variance is preferred for applications to which thisinvention particularly relates because, with local erosion of thecoating and/or pinholes or cracks therein, the exposed area ofsacrificial metal, as compared to the exposed area of substrate, may notbe expected to be large enough to offer satisfactory cathodic protectionif the potential difference between the two metals is less than about0.2 v. As noted above, however, it is not preferred to have thispotential difference substantially above the minimum because, in thatevent, the principal result is merely to accelerate the rate of loss ofthe protective metal by corrosion without increasing the amount ofsacrificial protection afforded. Generally, and as a practical matter,satisfactory results have been achieved in accordance herewith if thepotential difference between the substrate and the protective coating iswithin the range of about 0.3-0.7 v. for coating thicknesses within the1-2 mil range.

Considering the foregoing, the above electrode potential data indicatethat, whereas the aluminum-zinc coating noted above might givesubstantial sacrificial protection on AMS 5616 steel articles, it fallsfar below the protection afforded by the zinc-cadmium coatings on AMS6304 articles, undoubtedly because of the much lower proportion of zincin the coating and notwithstanding the fact that most of the zinc isconcentrated toward the substrate. Thus, although the aluminum-zinccoatings may offer enhanced overall protection against corrosion anderosion in saline atmospheres, such protection must largely beattributable to coating integrity or resistance other than cathodicprotection-as contrasted with the zinc-cadmium coatings where totalprotection is achieved first by the inherent resistance of thecadmium-rich surface and, thereafter, by sacrificial protection of thezinc-rich interior of the coating as the coating thickness becomeseroded or corroded away.

As will be apparent from the foregoing, there are provided in accordanceherewith coating materials and techniques for providing multicomponentdiffusion coatings on metal articles by simple, even one-step packcementation procedures for protection of the metal articles from erosiveand corrosive saline environments. The coatings embodying and forpracticing this invention are readily applied at temperatures low enoughso that the coating step does not induce crystallographic ormetallurgical transformations in the article being coated which mightaffect the mechanical properties thereof.

Furthermore, these multicomponent coatings are produced on the basemetal article in such fashion that the coatings become more anodictoward the substrate-coating interface for increased cathodicsacrificial protection of the substrate as the surface of the coatingsbecomes eroded away. Thus, the surface of the coating is rich in acomponent of increased corrosion and erosion resistance, while the innerportions of the coatings are rich in a component offering greatersacrificial protection of the substrate although that component may haveconsiderably less corrosion or erosion resistance. In any event, ifthere are components of the coating which are not anodic with respect tothe substrate, electrochemical contact between the substrate and suchless noble components is avoided despite the interdiffusion of allcomponents and the substrate for complete unification and integrationthereof against a possibility of fractures or spalling or separation ofcoating and substrate even under intense mechanical strain and thermalshock conditions.

While the methods and compositions set forth above form preferredembodiments of this invention, this invention is not limited to theseprecise methods and compositions, and changes may be made thereinwithout departing from the scope of this invention which is defined inthe appended claims.

What is claimed is:

ll. A ferrous metal article to be subjected in use to corrosion bysaline environments and erosion by abrasion and having thermallydiffused into the outer surface thereof a multicomponent diffusioncoating for both mechanical and cathodic protection of said articleagainst said erosion and corrosion, said multicomponent coatingcomprising a first coating metal of zinc which is both diffusiblysoluble in the metal of said article and substantially anodicelectrochemically with respect thereto as diffused into the surfacethereof and a second coating metal which is both less anodic than saidfirst coating metal with respect to said article and more resistant thansaid first coating metal to said saline corrosion, said second coatingmetal being selected from the group consisting of Al, Cd, Ni and Pb,said two coating metals being differentially concentrated throughout thethickness of said coating on said article forming a concentrationgradient throughout said coating whereby said first and more anodiccoating metal is concentrated adjacent the article-coating interfaceproviding enhanced sacrificial cathodic protection of said article aftersaid coating becomes partially eroded away by said abrasive erosion andsaid second coating metal is concentrated adjacent the outer surface ofsaid coating providing initial protection of said coated article againstsaid saline corrosion, both said coating metals and said article beingmetallurgically integrated together by thermal interdiffusion thereamongat the surface of said article.

2. A ferrous metal article to be subjected in use to corrosion by salineenvironments and erosion by abrasion and having thermally diffused intothe outer surface thereof a multicomponent diffusion coating for bothmechanical and cathodic protection of said article against said erosionand corrosion, said multicomponent coating comprising a first coatingmetal of zinc which is both diffusibly soluble in the metal of saidarticle and substantially anodic electrochemically with respect theretoas diffused into the surface thereof and a second coating metal which isboth less anodic than said first coating metal with respect said articleand more resistant than said first coating metal to said salinecorrosion, said second coating metal being selected from the groupconsisting of Al, Cd, Ni and Pb, said two coating metals beingdifferentially concentrated throughout the thickness of said coating onsaid article forming a concentration gradient throughout said coatingwhereby said first and more anodic coating metal is concentratedadjacent the article-coating interface providing enhanced sacrificialcathodic protection of said article after said coating becomes partiallyeroded away by said abrasive erosion and said second coating metal isconcentrated adjacent the outer surface of said coating providinginitial protection of said coated article against said saline corrosion,both said coating metals and said article being metallurgicallyintegrated together by thermal interdiffusion thereamong at the surfaceof said article, the electrode potential of the multicomponentprotective coating as measured against a standard calomel electrode in asaline environment being at least about 0.2 volts more anodic than thatof the uncoated surface of said ferrous metal article.

3. A coated ferrous metal article as recited in claim 2 in which theelectrode potential of said multicomponent protective coating is withinthe range of about 0.30.7 volts more anodic than that of the uncoatedsurface of said article.

2. A ferrous metal article to be subjected in use to corrosion by salineenvironments and erosion by abrasion and having thermally diffused intothe outer surface thereof a multicomponent diffusion coating for bothmechanical and cathodic protection of said article against said erosionand corrosion, said multicomponent coating comprising a first coatingmetal of zinc which is both diffusibly soluble in the metal of saidarticle and substantially anodic electrochemically with respect theretoas diffused into the surface thereof and a Second coating metal which isboth less anodic than said first coating metal with respect said articleand more resistant than said first coating metal to said salinecorrosion, said second coating metal being selected from the groupconsisting of Al, Cd, Ni and Pb, said two coating metals beingdifferentially concentrated throughout the thickness of said coating onsaid article forming a concentration gradient throughout said coatingwhereby said first and more anodic coating metal is concentratedadjacent the article-coating interface providing enhanced sacrificialcathodic protection of said article after said coating becomes partiallyeroded away by said abrasive erosion and said second coating metal isconcentrated adjacent the outer surface of said coating providinginitial protection of said coated article against said saline corrosion,both said coating metals and said article being metallurgicallyintegrated together by thermal interdiffusion thereamong at the surfaceof said article, the electrode potential of the multicomponentprotective coating as measured against a standard calomel electrode in asaline environment being at least about 0.2 volts more anodic than thatof the uncoated surface of said ferrous metal article.
 3. A coatedferrous metal article as recited in claim 2 in which the electrodepotential of said multicomponent protective coating is within the rangeof about 0.3-0.7 volts more anodic than that of the uncoated surface ofsaid article.